In general, floor structures in automotive vehicles are known which have a drive shaft and an exhaust pipe extending underneath the floor in a front to back direction. This type of floor structure includes a tunnel portion which is formed as a part of a floor, normally at a front midway portion of the floor in a front to rear direction. The cross section of the tunnel portion is usually a hat-shaped configuration protruding upwardly from the surface of the floor, since the essential function of the tunnel portion is to provide an accommodating space for the drive shaft and the exhaust pipe underneath the floor at an appropriate height and to provide a passenger compartment which is as roomy as possible.
For example, Japanese Laid-open Utility Model Application No. 01-87972 discloses such a conventional tunnel portion having a hat-shaped cross section, which is formed on a floor, and extending from a front end of the floor to a rear end. Moreover, in an advanced commercial purpose vehicle, the floor is made of a front floor panel and a rear floor panel, and the rear floor panel is formed stepwise against the front floor panel so that the rear floor panel extends above the front floor panel. In such a commercial vehicle, a hat-shaped tunnel portion is only formed in the front floor panel portion, since the upper height of the tunnel is almost the same as the height of the rear floor panel. As a result, the rear floor panel is formed in a plane configuration without being reinforced by the tunnel. As a result, the rear floor panel is not rigid enough, and the connection between the front floor panel and the rear floor panel is relatively weak.